This invention relates to laminated window panels and especially to windshields of aircraft which are subject to high impact forces caused, for example, by accidental contact with birds in flight at high speeds. Window panels and windshields have been made of laminated glass and plastic which provide good optical properties and are also adaptable to withstand the substantial temperature differentials and pressurization to which modern aircraft are subjected. The windshield surface also has been of a protective material for resisting abrasion, chemical deterioration and thermal damage. All of these properties are required for aircraft windshields along with the restriction that the thickness and, accordingly, the weight of the windshields be kept to a minimum.
Although laminated window panels and windshields of an acceptable weight as used heretofore for aircraft have substantially met these requirements, it has been found that they have not been able to withstand the impact from an accidental collision with a bird when the aircraft is traveling at a high rate of speed. At high altitudes, this problem does not exist because birds are not able to fly above a certain height; however, this is a serious problem when aircraft are flying at low altitudes which is necessary for landing and takeoff. It is also necessary for aircraft to fly at low altitudes in certain situations as, for example, in low altitude military aircraft maneuvers.
With the window panel constructions used heretofore, it has been necessary to increase the thickness and weight substantially to provide efficient protection for aircrew members from bird impacts, especially at high speeds of from 250 knots to 500 knots and above where the forces generated by the impact of a bird are great enough to damage a windshield of more acceptable thickness and weight and permit penetration by the bird into the cockpit of the aircraft. This has presented a problem which is most serious in view of the consequences of an accident of this type of modern high speed aircraft.